For polyethylene, and for high density polyethylene (HDPE) in particular, the molecular weight distribution (MWD) is a fundamental property which determines many properties of the polymer, and thus its applications. It is generally recognized in the art that the MWD of a polyethylene resin may principally determine the physical, and in particular the mechanical, properties of the resin and that the provision of different molecular weight polyethylene molecules may significantly affect the rheological properties of the polyethylene as a whole.
Since an increase in the molecular weight normally improves the physical properties of polyethylene resins, there is a strong demand for polyethylene having high molecular weight. For the purposes of this application, a high molecular weight polyethylene is one having a Mn of at least 1×105, typically from about 1×105 to about 1×107. However, it is the high molecular weight molecules which render the polymers more difficult to process. On the other hand, a broadening of the molecular weight distribution tends to improve the flow of the polymer when it is being processed at high rates of shear. Accordingly, in applications requiring a rapid transformation employing quite high throughputs of the material through a die, for example in blowing and extrusion techniques, the broadening of the molecular weight distribution permits an improvement in the processing of polyethylene at high molecular weight relative to a low melt index of the polyethylene, which is known in the art. It is known that when the polyethylene has a high molecular weight and also a broad molecular weight distribution, the processing of the polyethylene is made easier as a result of the low molecular weight portion and also the high molecular weight portion contributes to a good impact resistance. A polyethylene of this type may be processed utilizing less energy with higher processing yields.
A polymer comprising two groups of molecules with different average molecular masses is said to be bimodal. The manufacture of multimodal polymers is a basic challenge in the field of materials as polymers of this type make it possible to combine, in the same material, the properties of each group of molecules from which it is composed. For example, polymers of high mass introduce good mechanical strength, whereas low masses make it possible to retain, in the material, good fluidity at high temperature, which facilitates its processing.
As discussed above, the high molecular weight fraction provides good mechanical properties to the high density polyethylene and the low molecular weight fraction is required to give good processability to the high density polyethylene. The high molecular weight fraction having relatively high viscosity may lead to difficulties in processing such a high molecular weight fraction. In a bimodal high density polyethylene, the mixture of the high and low molecular weight fractions is adjusted as compared to a monomodal distribution to optimize both the quantity and the molecular weight of high molecular weight species in the polymer. This may provide improved mechanical properties and/or improved processability depending on the end use or the process used to fabricate the end use application.
It is accordingly recognized in the art that it is desirable to have a bimodal distribution of molecular weight in the high density polyethylene. For a bimodal distribution a graph of the MWD as determined for example by gel permeation chromatography may include, provided that the average molecular weight of the two species is sufficiently different, a “shoulder” on the high molecular weight side of the peak of the molecular weight distribution. A resin may have no discernable shoulder and still be bi-modal.
It is a continuing goal of the industry to produce polyethylene having improved properties, such as higher stiffness and higher environmental stress cracking resistance (ESCR) that are important considerations for applications such as pipes, large and small molded parts, and 55-gallon drums and the like.